The present invention relates generally to fuel cell systems and, more specifically, to fuel cell electrode pair and stack assemblies and various methods relating thereto.
A fuel cell is an energy conversion device that consists essentially of two opposing electrodes, an anode and a cathode, ionically connected together via an interposing electrolyte. Unlike a battery, fuel cell reactants are supplied externally rather than internally. Fuel cells operate by converting fuels, such as hydrogen or a hydrocarbon (e.g., methanol), to electrical power through an electrochemical process rather than combustion. It does so by harnessing the electrons released from controlled oxidation-reduction reactions occurring on the surface of a catalyst. A fuel cell can produce electricity continuously so long as fuel is supplied from an outside source.
In electrochemical fuel cells employing methanol as the fuel supplied to the anode (also commonly referred to as a xe2x80x9cDirect Methanol Fuel Cell (DMFC)xe2x80x9d system), the electrochemical reactions are essentially as follows: first, a methanol molecule""s carbon-hydrogen, and oxygen-hydrogen bonds are broken to generate electrons and protons; simultaneously, a water molecule""s oxygen-hydrogen bond is also broken to generate an additional electron and proton. The carbon from the methanol and the oxygen from the water combine to form carbon dioxide. Oxygen from air (supplied to the cathode) is likewise simultaneously reduced at the cathode. The ions (protons) formed at the anode migrate through the interposing electrolyte and combine with the oxygen at the cathode to form water. From a molecular perspective, the electrochemical reactions occurring within a direct methanol fuel cell (DMFC) system are as follows:                             ⁢                  Anode          ⁢                      :                                                      ⁢                                                            CH                3                            ⁢              OH                        +                                          H                2                            ⁢              O                                ->                                    6              ⁢                              H                +                                      +                          6              ⁢                              e                -                                      +                          CO              2                                                                    E          0                =                  0.04          ⁢                      xe2x80x83                    ⁢          V                                    vs        .                    NHE                      (        1        )                                        ⁢                  Cathode          ⁢                      :                                                      ⁢                                                            3                2                            ⁢                              O                2                                      +                          6              ⁢                              H                +                                      +                          6              ⁢                              e                -                                              ->                      3            ⁢                          H              2                        ⁢            O                                                        E          0                =                  1.23          ⁢                      xe2x80x83                    ⁢          V                                    vs        .                    NHE                      (        2        )                                        ⁢                  Net          ⁢                      :                                                      ⁢                                                            CH                3                            ⁢              OH                        +                                          3                2                            ⁢                              O                2                                              ->                                    2              ⁢                              H                2                            ⁢              O                        +                          CO              2                                                                    E          0                =                  1.24          ⁢                      xe2x80x83                    ⁢          V                                    vs        .                    NHE                      (        3        )            
The various electrochemical reactions associated with other state-of-the-art fuel cell systems (e.g., hydrogen or carbonaceous fuel) are likewise well known to those skilled in the art of fuel cell technologies.
With respect to state-of-the-art fuel cell systems generally, several different configurations and structures have been contemplatedxe2x80x94most of which are still undergoing further research and development. In this regard, existing fuel cell systems are typically classified based on one or more criteria, such as, for example: (1) the type of fuel and/or oxidant used by the system, (2) the type of electrolyte used in the electrode stack assembly, (3) the steady-state operating temperature of the electrode stack assembly, (4) whether the fuel is processed outside (external reforming) or inside (internal reforming) the electrode stack assembly, and (5) whether the reactants are fed to the cells by internal manifolds (direct feed) or external manifolds (indirect feed). In general, however, it is perhaps most customary to classify existing fuel cell systems by the type of electrolyte (i.e., ion conducting media) employed within the electrode stack assembly. Accordingly, most state-of-the-art fuel cell systems have been classified into one of the following known groups:
1. Alkaline fuel cells (e.g., KOH electrolyte);
2. Acid fuel cells (e.g., phosphoric acid electrolyte);
3. Molten carbonate fuel cells (e.g., Li2CO3/K2CO3 electrolyte);
4. Solid oxide fuel cells (e.g., yttria-stabilized zirconia electrolyte);
5. Proton exchange membrane fuel cells (e.g., NAFION electrolyte).
Although these state-of-the-art fuel cell systems are known to have many diverse structural and operational characteristics, such systems nevertheless share many common characteristics with respect to the joining or bonding together of the individual electrode structures that form the electrode stack assembly. Put simply, most conventional state-of-the-art electrode stack assemblies consist essentially of a series of conjoined anode and cathode structures, wherein the faces of the electrode structures (together with any separator and fluid flow plates) are adjacently positioned next to one another and attached together by means of adhesives and/or bolted tie rods. Moreover, most conventional fuel cell stack assemblies also include a plurality of fluid tight resilient seals, such as elastomeric gaskets. The use of such elastomeric gaskets (together with the disparate materials used for the separator and fluid flow plates) necessitates the need to have a constant compressive force applied along the longitudinal axis of the stack assembly to ensure resilient sealing. Hence, and in order to maintain proper sealing between adjacent surfaces, conventional fuel cell stacks are generally compressed together by one or more metal tie rods or tension members. In general, the end bolted tie rods or tension members of such conventional state-of-the-art stack assemblies extend through holes formed in stack""s end plates; in this configuration a constant compressive force is maintained throughout the stack assembly. In addition, adhesives such as, for example, epoxides are often also applied between the various opposing faces of the stack components to ensure that the stack is hermetically sealed.
Exemplary fuel cell electrode stack assemblies in accordance with the prior art are disclosed in U.S. Pat. No. 5,723,228 to Okamoto (discloses DMFC system having a series of bolted together membrane electrode assemblies with interposing gaskets and separator plates), (2) U.S. Pat. No. 6,190,793 B1 to Barton et al. (discloses a fuel cell stack assembly having non-conductive tie rod tension members); and (3) U.S. Pat. No. 6,057,053 to Gibb (discloses compression assembly for a fuel cell stack). A significant problem associated with these conventional fuel cell stack designs, however, is their limited ability to be scaled down so as to be manufacturable on a micro-scale basis. In particular, these conventional fuel cell stack designs are not generally amenable to the xe2x80x9cstackingxe2x80x9d of silicon and/or sol-gel derived electrode structures (which electrode structures are generally made by micro-fabrication techniques and are associated with micro-scale fuel cell systems). Accordingly, there is still a need in the art for improved fuel cell electrode stack assemblies, systems, and related methods. The present invention fulfills these needs and provides for further related advantages.
In brief, the present invention relates generally to fuel cell systems and, more specifically, to fuel cell electrode pair and stack assemblies and various methods relating thereto. In one embodiment, the present invention is directed to a fuel cell electrode pair assembly adapted for use with a fuel cell system, wherein the electrode pair assembly comprises an anode structure derived from a first silicon substrate and an opposing cathode structure derived from a second silicon substrate, wherein at least (i) the anode structure comprises one or more discrete anodic porous active regions disposed across a top surface, or (ii) the cathode structure comprises one or more discrete cathodic porous active regions disposed across a top surface, and wherein the anode structure and the cathode structure each have at least one adjoining support member made of silicon, one or more plastics, or one or more glasses, and wherein the at least one adjoining support member of the anode structure and the at least one adjoining support member of the cathode structure have interfacing surfaces that are bonded together with an optional interposing binding material and with at least one selectively positioned bond to thereby form a hermetic seal, wherein the at least one selectively positioned bond is selected from the group consisting of a silicon-metal eutectic-silicon bond, a silicon-frit-silicon bond, a silicon-metal-silicon microwave bond, a silicon-polymer adhesive-silicon bond, a silicon-polymer adhesive-plastic bond, a silicon-polymer adhesive-glass bond, or a silicon-glass anodic bond.
In some embodiments, the at least one adjoining support member of the anode structure or the at least one adjoining support member of the cathode structure is made of silicon that is integral to the anode structure or the cathode structure. In other embodiments the at least one adjoining support member of the anode structure and the at least one adjoining support member of the cathode structure are both made of silicon, and wherein the optional binding material is a metal, and wherein the at least one selectively positioned bond is the silicon-metal eutectic-silicon bond. Preferably, however, the metal is gold, tin, lead, copper, silver, aluminum, or a combination thereof, and more preferably the metal is gold. In addition, the fuel cell electrode pair assembly may further comprise (i) a dielectric layer on at least one of the interfacing surfaces, wherein the dielectric layer comprises silicon dioxide, silicon nitride, or a combination thereof, and (ii) an adhesion layer on the dielectric layer, wherein the adhesion layer comprises titanium, chromium, tungsten, aluminum, or a combination thereof. Preferably, however, the adhesion layer is a titanium-tungsten layer.
In other embodiments, the at least one adjoining support member of the anode structure and the at least one adjoining support member of the cathode structure are both made of silicon, and wherein the optional binding material is a frit, and wherein the at least one selectively positioned bond is the silicon-frit-silicon bond. Preferably, however, the frit comprises a silicate. In addition, the fuel cell electrode pair assembly may further comprise a dielectric layer on at least one of the interfacing surfaces, wherein the dielectric layer comprises silicon dioxide, silicon nitride, or a combination thereof.
In still other embodiments, the at least one adjoining support member of the anode structure and the at least one adjoining support member of the cathode structure are both made of silicon, and wherein the optional binding material is a metal, and wherein the at least one selectively positioned bond is the silicon-metal-silicon microwave bond. Preferably, however, the metal is gold, tin, lead, copper, silver, aluminum, or a combination thereof, and more preferably the metal is gold. In addition, the fuel cell electrode pair assembly may further comprise (i) a dielectric layer on at least one of the interfacing surfaces, wherein the dielectric layer comprises silicon dioxide, silicon nitride, or a combination thereof, and (ii) an adhesion layer on the dielectric layer, wherein the adhesion layer comprises titanium, chromium, tungsten, aluminum, or a combination thereof. Preferably, however, the adhesion layer is a titanium-tungsten layer.
In still further embodiments, the at least one adjoining support member of the anode structure and the at least one adjoining support member of the cathode structure are both made of silicon, and wherein the anode structure and the cathode structure are made of different materials, and wherein the optional binding material comprises a polymer adhesive, and wherein the at least one selectively positioned bond is the silicon-polymer adhesive-silicon bond. In these embodiments, the polymer adhesive may comprise one or more amorphous fluoropolymers, benzocyclobutane, polydimethylsiloxane, perfluoro 2,2-dimethyl-1,3-dioxole, tetrafluoroethylene, or a combination thereof.
In still further embodiments, the at least one adjoining support member of the anode structure and the at least one adjoining support member of the cathode structure are each made of silicon or the one or more plastics, and wherein the anode structure and the cathode structure are made of different materials, and wherein the optional binding material comprises a polymer adhesive, and wherein the at least one selectively positioned bond is the silicon-polymer adhesive-plastic bond. In these embodiments, the one or more plastics are each independently selected from the group consisting of siloxanes, epoxies, polyimides, polyphenylene ethers, polyphenylene sulfides, polysufones, or fluoropolymers; and the polymer adhesive may comprise one or more amorphous fluoropolymers, benzocyclobutane, polydimethylsiloxane, perfluoro 2,2-dimethyl-1,3-dioxole, tetrafluoroethylene, or a combination thereof.
In still further embodiments, the at least one adjoining support member of the anode structure and the at least one adjoining support member of the cathode structure are each made of silicon or the one or more glasses, and wherein the anode structure and the cathode structure are made of different materials, and wherein the optional binding material comprises a polymer adhesive, and wherein the at least one selectively positioned bond is the silicon-polymer adhesive-glass bond. In these embodiments, the one or more glasses may comprise a borosilicate glass. In addition, the polymer adhesive may comprise one or more amorphous fluoropolymers, benzocyclobutane, polydimethylsiloxane, perfluoro 2,2-dimethyl-1,3-dioxole, tetrafluoroethylene, or a combination thereof.
In still further embodiments, the at least one adjoining support member of the anode structure and the at least one adjoining support member of the cathode structure are each made of silicon or the one or more glasses, and wherein the anode structure and the cathode structure are made of different materials, and wherein the at least one selectively positioned bond is the silicon-glass anodic bond. In these embodiments, the one or more glasses may comprise a borosilicate glass.
These and other aspects of the present invention will become more evident upon reference to the following detailed description and attached drawings. It is to be understood, however, that various changes, alterations, and substitutions may be made to the specific fuel cell electrode structures disclosed herein without departing from the essential spirit and scope of the present invention. In addition, it is to be further understood that the drawings are illustrative and symbolic representations of exemplary embodiments of the present invention (hence, they are not necessarily to scale). Finally, it is expressly provided that all of the various references cited herein are incorporated by reference in their entireties.